Many coal liquefaction processes which are at an advanced stage of development operate in the liquid phase and are designed to produce boiler fuels (heavy liquids and solids at room temperature). The raw effluent product from the liquefaction stage of these processes consists of a very heavy, viscous liquefied coal with solids suspended in it. The coal minerals content of the product will be about 8-12%, and this has to be reduced to about 0.15% to meet stack emission requirements.
The inorganic content of coal represented by minerals, sulfur, arsenic, and other trace elements in the inorganic phase of coal present problems, since these elements can poison the catalysts utilized in coal gasification and liquefaction processes. Separation of these inorganics, especially sulfur, from the organic matter prior to use as fuel or feedstock has the advantage of reducing the amount of desulfurization, and no ash or slag or other extraneous material is left in the coal reactor. A pure carbon or hydrocarbon feedstock can be directly converted into various end products. Furthermore, raw coals vary widely in mineral keep and sulfur keep. As previously stated, such coals can not be used directly in many chemical conversion processes, e.g. coal liquefaction, where these impurities interfere with catalyst activity, and even with the processes in advanced stage of development the yields vary widely.
Solvent extraction of coal can be conducted in a manner that separates some of the inorganics such as minerals from a coal extract. However, current economic analysis indicates that since the yield of solvent extract is fairly low, typically around 20% or less, and the cost of the solvent is high, the residual coal must be separated from residual solvent and recovered and utilized as fuel. Though there have been many attempts to increase the amount of extraction, none of the processes for solvent extraction being developed have approached an economic level.
Extraction of the organic matter in coal with near or supercritical solvents is another possible way to perform the upgrading of coal into an inorganic-free, organic extract. Near the critical temperature, a highly dense gas which acts like a liquid solvent and efficiently extracts soluble components from coal can be made. Near the critical temperature the dense gas solvent can expand and contract with the involvement of very little energy. On expansion the dense gas loses its solvent properties, rendering it very easy and economical to separate solute from the dense solvent gas. Separation of the dense gas from the coal followed by expansion to release the solute is much easier than separating solvent liquid from a solid and a solute, as must be done in conventional solvent processes. The current state of the art for process demonstration units is exemplified in the publication of R. R. Maddocks et al in C. E. Progress (CEP), June, 1979 at pages 49-55. Maddocks et al report the extraction of coal at the critical temperature of toluene, 319.degree. C. (606.degree. F.), provides 21 weight percent yield of extract having a low ash content (0.08%) from coal having nearly 6.1% ash content. It is further suggested that this process would be commercially attractive if yields of extract could be increased. Other studies have shown that the economics of coal extraction rapidly rise as the extraction and efficiency increases from 20 to 40%.
It would be expected that coal extraction efficiency under supercritical conditions would increase if more efficient coal solvents such as the bent ring coal extract type of solvents such as phenanthrene were utilized. However, when phenanthrene or other highly efficient solvents are utilized, the critical temperature is so high that the coal extract cross-links or polymerizes and forms a very high percentage of char, or the temperature is sufficiently high to gasify the coal. Furthermore, very expensive materials are required for constructing a higher temperature reactor, and the energy cost to operate a higher temperature reactor and the energy cost to operate a higher temperature process is significantly higher. Other examples of low yield, supercritical solvent extraction of coal are U.S. Pat. Nos. 3,558,468 and 3,607,717 which illustrates the use of an aromatic solvent such as benzene at supercritical conditions. U.S. Pat. No. 3,607,716 utilizes phenanthrene under supercritical conditions to upgrade extracts of coal. U.S. Pat. No. 4,036,731 discloses a mixed solvent, hydrogen-active supercritical extraction process utilizing an aromatic solvent with a hydrogen donor solvent such as tetralin, tetrahydroquinoline or o-cyclohexylphenol, preferably practiced in the presence of hydrogen gas. Again the yields are mainly in the 20% range with some yields in the 30% range.